Electron multibeam technology for
mask and wafer writing at 0.1 nm
address grid
Elmar Platzgummer
Christof Klein
Hans Loeschner
Downloaded From: http://nanolithography.spiedigitallibrary.org/ on 08/22/2013 Terms of Use: http://spiedl.org/terms
J. Micro/Nanolith. MEMS MOEMS 12(3), 031108 (Jul–Sep 2013)
Electron multibeam technology for mask and wafer
writing at 0.1 nm address grid
Elmar Platzgummer
Christof Klein
Hans Loeschner
IMS Nanofabrication AG
Schreygasse 3
A-1020 Vienna, Austria
E-mail: Hans.Loeschner@ims.co.at
Abstract. IMS Nanofabrication realized a 50 keV electron multibeam
proof-of-concept (POC) tool confirming writing principles with 0.1 nm
address grid and lithography performance capability. The POC system
achieves the predicted 5 nm 1 sigma blur across the 82 μm × 82 μm
array of 512 × 512 (262,144) programmable 20 nm beams. 24-nm half
pitch (HP) has been demonstrated and complex patterns have been written in scanning stripe exposure mode. The first production worthy system
for the 11-nm HP mask node is scheduled for 2014 (Alpha), 2015 (Beta),
and first-generation high-volume manufacturing multibeam mask writer
(MBMW) tools in 2016. In these MBMW systems the max beam current
through the column is 1 μA. The new architecture has also the potential for
1× mask (master template) writing. Substantial further developments are
needed for maskless e-beam direct write (EBDW) applications as a beam
current of >2 mA is needed to achieve 100 wafer per hour industrial targets for 300 mm wafer size. Necessary productivity enhancements of
more than three orders of magnitude are only possible by shrinking the
multibeam optics such that 50 to 100 subcolumns can be placed on
the area of a 300 mm wafer and by clustering 10 to 20 multicolumn
tools. An overview of current EBDW efforts is provided. © The Authors.
Published by SPIE under a Creative Commons Attribution 3.0 Unported License.
Distribution or reproduction of this work in whole or in part requires full attribution of the original
publication, including its DOI. [DOI: 10.1117/1.JMM.12.3.031108]
Subject terms: electron; multibeam; mask writer; template writer; direct wafer writer;
multibeam mask writer; e-beam direct write.
Paper 13043SSP received Apr. 9, 2013; revised manuscript received Jun. 11, 2013;
accepted for publication Jul. 3, 2013; published online Aug. 2, 2013.
1 Introduction
There is increased industrial interest and demand for electron
beam lithography (EBL) in order to provide (1) a fast multibeam mask writer (MBMW) for the realization of leadingedge 4× masks1 and 1× templates2 and (2) maskless electron
beam direct write (EBDW) on 300 and 450 mm wafers,3 in
particular for cutting lithography.4,5
For mask writing, 50 keV single variable-shaped beam
(VSB) writing tools are available with a current density as
high as 400 A∕cm2 (Ref. 6), providing ca. 0.1 μA average
current. For sub-14-nm half pitch (HP) mask technology
nodes, however, single VSB technology cannot keep up
with the exponential growth of shot numbers at substantial
reduced average shot size.7 Further, there is the need to
enhance the resist exposure dose by a factor of 5 to 10 up
to 100 μC∕cm2 in order to ensure sufficiently low line
edge and line width roughness.7,8
There are several proposals on how to realize a breakthrough multibeam (MB) column solution that can provide
10× to 40× more beam current than the most advanced VSB
tools (Fig. 1): (1) A slim (ca. 20 mm diameter) column providing a fixed-shape beam as proposed by Multibeam
Corporation for complementary electron beam lithography
with the argument that cutting needs to be done on 5% of
the 300 mm wafer area and a single beam shape is sufficient.9
(2) A multicolumn cell approach as pursued by Advantest
using VSB combined with character projection in each
cell to improve throughput.10 (3) A multiple (variable-)
shaped beam column as proposed by Vistec.11 (4) The
J. Micro/Nanolith. MEMS MOEMS
IMS Nanofabrication column with a blanking device and
projection optics with 200× reduction.12 (5) A reflective
electron beam lithography (REBL) configuration as pursued
by KLA-Tencor.13
In order to reach an electron beam current of >2 mA
beam current as needed is for 100 wafer per hour (WPH)
EBDW, an additional productivity enhancement by more
than three orders of magnitude is needed as shown in
Fig. 2. First, a 1 to 4 μA MB column needs to be shaped
sufficiently small so that 50 to 100 columns can be placed
on the area of a 300 mm wafer and thus ca. 200 μA beam
current can be achieved. Second, 10 to 20 multicolumn tools
need to be clustered to realize the targeted >2 mA total beam
current required for 100 WPH EBDW.
For REBL, there is the KLA-Tencor proposal13 to realize
a multicolumn tool configuration with 36 columns exposing
six wafers in parallel, and to cluster such tools to reach the
100 WPH EBDW target (Fig. 3). In the MAPPER approach,
there is the target to realize 13,200 microcolumns within an
area of 26 mm × 26 mm and to have 49 programmable
beams within each microcolumn, which hit the wafer substrate at 5 keV beam energy.14 With each microcolumn providing 13 nA, the targeted multi-microcolumn tool current is
ca. 170 μA. There is the target to cluster 10 microcolumn
tools to reach 1.7 mA beam current.
It should be pointed out that presently all efforts cited
above are concentrated on realizing a break-through MB column, i.e., to demonstrate writing performance with 1 to 4 μA
total beam current.
031108-1
Downloaded From: http://nanolithography.spiedigitallibrary.org/ on 08/22/2013 Terms of Use: http://spiedl.org/terms
Jul–Sep 2013/Vol. 12(3)
Platzgummer, Klein, and Loeschner: Electron multibeam technology for mask and wafer writing. . .
Fig. 1 Overview of proposed multibeam configurations. (a) SB: spot beam, (b) VSB: variable shaped beam, (c) CEBL: complementary electron
beam lithography, (d) MCC: multi-column cell, (e) MSB: mutiple variable shaped beam, (f) eMET: electron mask exposure tool, (g) PML2: projection
mask-less lithography, (h) REBL: reflective electron beam lithography, and (i) MAPPER: genuine name.
Fig. 2 Evolution of electron beam systems as needed to realize 100 WPH EBDW.
Multibeam multiaxis
Multibeam multicolumn
KLA-Tencor
REBL
IMS nanofabrication
PML2
Reflective electron
beam lithography
Projection maskless
lithography
MAPPER lithography
cluster array
10 units of ca 13,200 Micrcolumns
(49 beams / Micrcolumn) in parallel
Fig. 3 Schematics of REBL, PML2, and MAPPER multicolumn/cluster tool configurations.
For leading-edge mask exposures, already 1 μA beam current is sufficient to meet the industrial needs of realizing 10 h
write time even when using a resist with an exposure dose of
100 μC∕cm2 .8 Therefore, IMS Nanofabrication concentrates
efforts on development and realization of a multibeam mask
writer called electron mask exposure tool (eMET). A proofof-concept tool (eMET POC) was realized in 2011 (Ref. 8)
with extensive testing throughout 2012.15–17
2 eMET Principles and Realized Proof-of-Concept
Tool
The basic principles common to all eMET systems are shown
in Fig. 4. Electrons are extracted at gun-level first pass
J. Micro/Nanolith. MEMS MOEMS
through a multielectrode stack, which acts as a condenser
and generates a broad, homogeneous beam of ca. 25 mm
in diameter. This electron beam then impinges perpendicularly onto a programmable aperture plate system (APS),
where 512 × 512 (262,144) micrometer-sized beams are
formed (cf. 256 k-APS). Additionally, each beam can be
deflected individually by CMOS-controlled microdeflectors.
All beams (deflected and undeflected) then enter the projection optics of the system where they get accelerated from 5 to
50 keV beam energy in an electrostatic multielectrode lens
and 200× demagnified by a magnetic lens system located at
the bottom of the optical column. Only undeflected beams
make it to the substrate level. Deflected beams are filtered
031108-2
Downloaded From: http://nanolithography.spiedigitallibrary.org/ on 08/22/2013 Terms of Use: http://spiedl.org/terms
Jul–Sep 2013/Vol. 12(3)
Platzgummer, Klein, and Loeschner: Electron multibeam technology for mask and wafer writing. . .
Electron source
multielectrode
condenser optics
Aperture plate
Blanking plate
multielectrode
accelerating lens
Electron beam
projection optics
Stopping plate at
2nd cross-over
Beam steering multipole
mask blank
Scanning stage
Fig. 4 eMET principles.
out at a stopping aperture plate in the projection optics. All
eMET systems are designed to print 82-μm-wide stripes onthe-fly, i.e., the substrate is continuously moving at
constant velocity underneath the column while beams are
switched on and off according to the data fed into the
Fig. 5 eMET POC during factory acceptance tests at IMS
Nanofabrication, Vienna, Austria, and main specifications.
APS via the eMET data path. In this write mode, IMS’s proprietary writing strategy provides an inherent redundancy of
up to 16×. This high redundancy level averages out the
effects of individual defective beams and thereby allows
ignoring up to 100 (statistically distributed) defective
beams. Furthermore, the operability of APS units is monitored in situ on a weekly basis by measurements that give
information on the precise position of all defective beams.
This information can be used to further reduce the impact
of defective beams via online correction in the eMET data
path. This inherent property of the eMET writing strategy
coupled with the online correction capability drastically
reduces the technical risk associated with the most critical
element of this new technology and speaks for the suitability
of IMS’s MB solution for high-volume manufacturing (HVM).
In 2011 a first POC version—the eMET POC—was integrated and finalized based on a tight schedule. The eMET
POC column was designed from scratch to meet all lithographic requirements of the 11-nm HP node such as
Fig. 6 (a) Simulated change of edge position in 0.1 nm steps showing several examples between 30.0 and 40.0 nm. (b) Simulated deviation of
edge position from 0.1 nm address grid versus line width. (c) Simulated change of edge with 10% change of dose versus line width.
J. Micro/Nanolith. MEMS MOEMS
031108-3
Downloaded From: http://nanolithography.spiedigitallibrary.org/ on 08/22/2013 Terms of Use: http://spiedl.org/terms
Jul–Sep 2013/Vol. 12(3)
Platzgummer, Klein, and Loeschner: Electron multibeam technology for mask and wafer writing. . .
Fig. 7 eMET POC exposure of 50 nm lines whose pitch was varied between 100.0 and 109.9 nm in 0.1 nm steps; vertical lines in HSQ negative
resist and horizontal lines in pCAR positive resist. The line width and pitch values are written with 30 nm line width.
resolution capability (i.e., minimum feature size), pattern
fidelity, critical dimension uniformity (CDU), line width
roughness (LWR), registration, etc., within 1 cm × 1 cm
fields on 6 in. mask blanks. The fully integrated eMET
POC during 2012 factory acceptance tests at IMS
Nanofabrication in Vienna, Austria, can be seen in the left
of Fig. 5.
Using 0.25 μm CMOS technology, a blanking plate
was realized with 512 × 512 ¼ 262;144 apertures of 4 μm ×
4 μm opening size and 32 μm pitch between the apertures
within a ca. 16.4 mm × 16.4 mm field. With 200× reduction
of the electron projection optics, 256 k (k ¼ 1024) programmable beams of 20 nm beam size are projected at 50 keV
beam energy to the 6 in. mask substrate within a ca. 82 μm ×
82 μm beam array field. Monitor exposures were done on
J. Micro/Nanolith. MEMS MOEMS
64 shots per dot
5nm
physical grid
40nm dots with 80nm pitch
LCDU = 1.63nm 3sigma
Every dot on equivalent physical grid position
40nm dots with 81nm pitch
LCDU = 1.61nm 3sigma
Every fifth dot on equivalent physical grid position
Fig. 8 eMET POC exposure of 40 nm dots in HSQ negative resist with
80 nm pitch (above) and 81 nm pitch (below).
031108-4
Downloaded From: http://nanolithography.spiedigitallibrary.org/ on 08/22/2013 Terms of Use: http://spiedl.org/terms
Jul–Sep 2013/Vol. 12(3)
Platzgummer, Klein, and Loeschner: Electron multibeam technology for mask and wafer writing. . .
50nm HP
40nm HP
Fig. 9 eMET POC exposure of 50 nm HP dots and 40 nm HP dots in
HSQ negative resist with improved corner rounding.
Fig. 11 eMET POC exposure of aggressive OPC mask pattern in
pCAR positive resist.
resist-coated 150-mm Si wafers. With this novel electron MB
optics, a very low column blur of ca. 5 nm 1 sigma was verified.8 Very low resist blur was added when using hydrogen
silsesquioxane (HSQ) negative resist, which needs a very
high dose of ca. 1000 μC∕cm2 .
First eMET POC results are reported here with exposures
on 6 in. mask blanks in a production worthy insensitive positive chemically amplified resist (pCAR).
3 eMET POC Exposure with 0.1 nm Address Grid
Using multiple exposure shot addressing (MESA) techniques17 with 20-nm beam size, there is the possibility,
using overlapping shots, to expose on a 5 nm physical grid
such that the line edge can be placed on a 0.1 nm address grid
[Fig. 6(a)] with deviations as small as 50 pm [Fig. 6(b)].
The simulated exposure latitude with respect to edge position
is 1.06 0.02 nm for 10% change of dose as shown in
Fig. 7(c).
A rigorous experimental study was done by printing
50 nm vertical and horizontal lines with pitch values varied
between 100.0 and 109.9 nm in steps of 0.1 nm. Such exposures were done in HSQ negative resist11 as well as in pCAR
positive resist (Fig. 7). In both cases there is a linear relationship between critical dimension-secondary electron microscope measured pitch versus design pitch with three
sigma deviations as low as 0.23 and 0.30 nm, respectively.
The CD value three sigma variations are 1.6 and 1.5 nm,
respectively. These experimental results demonstrate the
capability of MB writing with 0.1 nm address grid.
A further study of MB printing was done by exposing
40 nm dots in HSQ negative resist (1) with 80 nm pitch
and (2) with 81 nm pitch.17 There is no change of the
1.6 nm three sigma local CDU value when placing the
dots at grid positions different from the 5 nm physical
grid as shown in Fig. 8.
There is the possibility to realize improved corner rounding (Fig. 9) by placing serifs at the corners (Fig. 10).
According to MESA techniques, there is no throughput
degradation when inducing such pattern exposure
improvements.
4 eMET POC Exposure of Optical Proximity
Correction and Inverse Lithography Technology
Patterns
There is agreement between simulation and exposure results
of aggressive optical proximity correction (OPC) mask patterns in pCAR positive resist as demonstrated in Figs. 11
and 12.
This holds also for the exposure of inverse lithography
technology (ILT) mask patterns in HSQ negative and
pCAR positive resist (Fig. 13).
Fig. 10 Bit map (above) and 50-nm HP dot exposure (below) in HSQ negative resist without (left) and with (right) improved corner rounding.
J. Micro/Nanolith. MEMS MOEMS
031108-5
Downloaded From: http://nanolithography.spiedigitallibrary.org/ on 08/22/2013 Terms of Use: http://spiedl.org/terms
Jul–Sep 2013/Vol. 12(3)
Platzgummer, Klein, and Loeschner: Electron multibeam technology for mask and wafer writing. . .
Fig. 12 eMET POC exposure of aggressive OPC mask pattern in
pCAR positive resist.
5 eMET POC Resolution Using 20 nm Beam Size
The eMET POC resolution capability, using 20-nm beam
size, is shown in Fig. 14 with examples of 30-nm HP
45 deg ∕135 deg as well as 24 nm-HP and iso lines in
HSQ negative and pCAR positive resist. It should be
noted that HSQ resist, needing more than 1000 μC∕cm2
exposure dose, is useful for test purposes only, whereas
the insensitive pCAR resist is fulfilling advanced mask writing industrial needs.
It is straightforward to change the beam size to, e.g.,
10 nm by using an aperture plate (Fig. 4) with 2 μm×
2 μm openings. (For Beta and HVM tools, there will be
the possibility of in situ change of beam size, as outlined
in Ref. 8.) Due to the small column blur (5 nm one sigma),
the very small forward scattering of 50 keV electrons in
resist materials, and the small resist blur of, e.g., insensitive pCAR resist, there is expectation that using 10-nm
beam size with a resolution of 12-nm HP can be achieved.
To achieve sub-10-nm HP resolution, there are possibilities
to lower the aberration blur of the column optics with proprietary measures. Thus, as the Coulomb interaction blur is very
small, a total column blur below 5 nm one sigma will become
possible. To realize sub-10-nm HP resolution, a smaller beam
size of 8 and 5 nm, respectively, will be used.
6 eMET Multibeam Mask Writer Roadmap
There are stringent International Technology Roadmap for
Semiconductors requirements to lower the three sigma
LWR for future mask technology nodes. This can only be
accomplished by enhancing the resist exposure dose (dose
to size for dense 1∶1 line patterns) from 50 to
100 μC∕cm2 for the 11-nm HP mask technology node
and below (Fig. 15).
The eMET roadmap for MBMW tools is outlined in
Table 1. The realized MB column is used for Alpha,
Beta, and first-generation HVM mask writer tools, providing
256 k (k ¼ 1024) programmable beams for MESA-based MB
writing along 82-μm-wide stripes at constant stage velocity.
Fig. 13 eMET POC exposure of ILT mask pattern in HSQ negative and pCAR positive resist. The ILT test pattern was provided by Dai Nippon
Printing18,19 and was exposed according to design and also with two-times shrink.
J. Micro/Nanolith. MEMS MOEMS
031108-6
Downloaded From: http://nanolithography.spiedigitallibrary.org/ on 08/22/2013 Terms of Use: http://spiedl.org/terms
Jul–Sep 2013/Vol. 12(3)
Platzgummer, Klein, and Loeschner: Electron multibeam technology for mask and wafer writing. . .
Fig. 14 :eMET POC resolution (a) 45 deg and −45 deg 30-nm HP in HSQ negative (left) and pCAR positive resist (right). (b) Vertical and horizontal
24-nm HP in HSQ. (c) Any angle 24-nm iso lines in HSQ and pCAR (angle numbers written with 30 nm line width).
The eMET Alpha tool would be realized in 2014, integrating
the column with a production worthy platform and stage.
eMET Beta tools would be delivered in 2015 and first-generation HVM tools are scheduled in 2016.
Fig. 15 Monte Carlo simulation of the 3 sigma line width roughness
for 30 nm line width versus resist exposure dose. Parameter is the
combined tool and resist 1 sigma blur. The optimum total 1 sigma
blur is between 5 and 7.5 nm, meeting the requirements for the 6nm HP mask technology node when using a resist exposure dose
of 100 μC∕cm2 .
J. Micro/Nanolith. MEMS MOEMS
7 Summary
eMET POC MB writing is demonstrated with 24-nm HP resolution and a possibility to realize complex OPC and ILT
mask patterns on a 0.1 nm address grid.
Using an insensitive pCAR positive resist, the resolution
capability and throughput potential is verified for the 11-nm
HP mask technology node and below, where a resist exposure dose of 100 μC∕cm2 is required.
An eMET Alpha tool is scheduled for 2014. MBMW
Beta tools are scheduled for 2015, and first HVM tools
for 2016.
Productivity enhancements for 100 WPH EBDW are technically possible, but remain a great challenge. In addition to
compact multicolumn tool development, substantial investments in data path and system engineering are required.
031108-7
Downloaded From: http://nanolithography.spiedigitallibrary.org/ on 08/22/2013 Terms of Use: http://spiedl.org/terms
Jul–Sep 2013/Vol. 12(3)
Platzgummer, Klein, and Loeschner: Electron multibeam technology for mask and wafer writing. . .
Table 1 eMET multibeam mask writer roadmap.
POC
ALPHA
BETA
1st gen. HVM
2012
2014
2015
2016
Technology
node
Test: 11 nm HP
(7 nm logic)
11 nm HP
(7 nm logic))
11 nm HP
(7 nm logic)
11 nm HP
(7 nm logic)
Beam array
field
82 μm × 82 μm
82 μm × 82 μm
82 μm × 82 μm
82 μm × 82 μm
262,144
262, 144
262, 144
262, 144
Max current
(all beams “on”)
0.1 to 1 μA
1 μA
1 μA
1 μA
Throughput
ð≥ 100 μC∕cm2 Þ
< 10 cm2 ∕h
15 h/mask
10 h/mask
10 h/mask
# Beams
References
1. M. Chandramouli et al., “Future mask writers requirements for the sub
10 nm node era,” Proc. SPIE 8522, 85221K (2012).
2. M. Melliar-Smith, “The future of lithography—back to single patterning? or the rise of DSA & multiple patterning?,” presented at Joint Panel
at SPIE Advanced Lithography 2013, San Jose, California (2013).
3. B. J. Lin, “Future of multiple-e-beam direct-write systems,” J. Micro/
Nanolith. MEMS MOEMS 11(3), 0033011 (2012).
4. Y. Borodovsky, “EUV, EBDW—ArF replacement or extension,” presented at KLA-Tencor User Lithography Forum, San Jose, California
(2010).
5. H. Yaegashi et al., “Sustainability of double patterning process for lithographic scaling,” presented at SEMATECH 2011 Int. Symp. on
Lithography Extensions, Miami, Florida (2011).
6. S. Yoshitake et al., “EBM-8000: EB mask writer for product mask fabrication of 22-nm half-pitch generation and beyond,” Proc. SPIE 8166,
81661D (2011).
7. S. H. Lee et al., “The requirements for the future e-beam mask writer;
statistical analysis of pattern accuracy,” Proc. SPIE 8166, 81661B
(2011).
8. E. Platzgummer, C. Klein, and H. Loeschner, “eMET POC: realization
of a proof-of-concept 50 keV electron multibeam mask exposure tool,”
Proc. SPIE 8166, 816622 (2011).
9. D. Lam, D. Liu, and T. Prescop, “E-beam direct write (EBDW) as complementary lithography,” Proc. SPIE 7823, 78231C (2010).
10. A. Yamada and Y. Ooae, “Cell projection use and multi column
approach for throughput enhancement of EBDW system,” Proc.
SPIE 7823, 78231H (2010).
11. M. Slodowski et al., “Multi shaped beam: development status and
update on lithography results,” Proc. SPIE 7970, 79700E (2011).
12. E. Platzgummer, “Maskless lithography and nanopatterning with
electron and ion multi-beam projection,” Proc. SPIE 7637, 763703
(2010).
13. R. Freed et al., “Reflective electron-beam lithography performance for
the 10 nm logic node,” Proc. SPIE 8622, 86221J (2012).
14. M. J. Wieland et al., “Throughput enhancement technique for MAPPER
maskless lithography,” Proc. SPIE 7637, 76371Z (2010).
15. C. Klein, H. Loeschner, and E. Platzgummer, “50-keV electron multibeam mask writer for the 11-nm HP node: first results of the proof-ofconcept electron multibeam mask exposure tool,” J. Micro/Nanolith.
MEMS MOEMS 11(3), 031402 (2012).
16. E. Platzgummer, C. Klein, and H. Loeschner, “Results of proof-of-concept 50 keV electron multi-beam mask exposure tool (eMET POC),”
Proc. SPIE 8441, 8441–8458 (2012).
17. E. Platzgummer, C. Klein, and H. Loeschner, “Printing results of proofof-concept 50 keV electron multi-beam mask exposure tool (eMET
POC),” Proc. SPIE 8522, 852252 (2012).
18. N. Hayashi, “Computational lithography and challenges for mask making,” presented at LithoVision 2012, San Jose, California (2012).
19. N. Hayashi, “NGL mask readiness,” presented at The ConFab 2012
Conf., Las Vegas, Nevada (2012).
J. Micro/Nanolith. MEMS MOEMS
Elmar Platzgummer received a PhD in surface physics from the Vienna University of
Technology. He joined IMS Nanofabrication
in 1999 and has taken a key role in the development of IMS’s multibeam technology and
its application fields. He has generated
more than 30 patents and 100 publications.
In Oct 2005 he became chief technology officer and chief operating officer, and since Oct
2012, he is chief executive officer of IMS
Nanofabrication AG.
Christof Klein received a PhD in surface science from Vienna University of Technology in
2003. In 2004 to mid 2006 he was Erwin
Schroedinger fellow at the National Center
for Electron Microscopy of the Lawrence
Berkeley National Laboratory. In August
2006 he joined IMS Nanofabrication where
he was director of Strategic Programs.
Hans Loeschner received a PhD in experimental physics from the University of Vienna.
He is a cofounder of IMS Nanofabrication and
VP Technical Marketing. He is coinventor of
more than 20 patents and author/coauthor of
more than 200 publications. In September
2012 he received the MNE Fellowship
Award for the advancement of electron and
ion beam technologies.
031108-8
Downloaded From: http://nanolithography.spiedigitallibrary.org/ on 08/22/2013 Terms of Use: http://spiedl.org/terms
Jul–Sep 2013/Vol. 12(3)